This invention relates to a process for separating water from, or dewatering, aggregate solid substrates which contain significant amounts of water. The method has specific relevance to fossilized biomass materials such as peat and coal but also is applicable to other biomass substrates such as agricultural and forestry residues (bagasse, corn stover, sawdust, etc.), crops deliberately grown for their fuel or chemical value, and sewage sludge. The present invention provides an energy efficient method for removing water from these materials prior to their ultimate use or interim conversion. Since the production of energy or fuel is generally the ultimate use for these substrates, an energy efficient dewatering process could be mandatory for those containing a relatively great amount of water.
It is quite likely that the invention concept will be applied to solid/liquid separations other than those described above since nothing in the concept of plug flow contacting restricts its use to water or aqueous fluids. Because of the relatively great amount of energy required to evaporate water, compared for example, to hydrocarbons, dewatering processes benefit most by use of the invention rather than evaporative or other conventional separation methods.
Water laden materials can often be dewatered by compression, the applicability of which is dependent upon the solid structure of the material. The predominant effect of compressive loading on water laden substrates of the type considered here is compaction of the solid structure which reduces the void volume available to the water. Material with a rigid structure, such as a particulate mixture with a wide size distribution, is usually difficult to compress. Peat, for example, can be economically dewatered down only to about 75% moisture content by mechanical squeezing.
A commonly used method of dewatering material such as filter cakes is air displacement. This method is most applicable to materials with an open and uniform pore structure. Where there is a wide particle and hence pore size range, it is very difficult to displace water from the finer void spaces using air because of the tendency for bypassing through the larger channels.
Nonaqueous liquid displacement of water from a porous material has been used commercially only rarely and then with materials that have high value compared to the biomass and similar type substrates considered here. The main drawback to solvent dewatering is the high cost of solvent recovery and/or loss relative to the product. Our copending patent application, Organic Solid Substrate Dewatering Process Utilizing Primary and Secondary Solvents, Ser. No. 280,264, filed July 2, 1981, now U.S. Pat. No. 4,339,882, typifies interest in liquid dewatering brought about by the high energy cost of conventional dewatering methods.
Liquid extraction, which is the commonly used and well understood separation process most similar to dewatering by solvent displacement, occurs by bringing about contact between differing liquid phases (one possibly intermixed or associated with a solid) so that a redistribution of the components in the original phases will occur. Design of contacting devices for liquid extraction is based on the definition that an equilibrium distribution, the ultimate product of perfectly intimate contacting, will be achieved from a single perfect contacting stage, (see for example, Treybal, Mass Transfer Operations, Ch. 10 - Liquid Extraction). It is well known, however, that for packed columns, such as are commonly used in chromatographic equipment, much better separation can be achieved than would be expected if one assumed the column to be a single equilibrium stage. This is explained in chromatographic theory by showing that the moving zone of contact does not reach equilibrium in the sense generally used in chemical engineering unit operations. This invention demonstrates that a similar phenomena can be used in a liquid displacement dewatering process, especially when used in combination with an energy efficient liquid separation method.
In our copending patent application, Organic Solid Substrate Dewatering Process Based on Plug Flow Contact by an Extractive Fluid, Ser. No. 327,610, filed Dec. 4, 1981, now U.S. Pat. No. 4,397,100, we disclosed the advantages which are achieved in the dewatering of organic solid substrates by contact with an extractive fluid when the organic solid substrate phase and the extractive fluid phase are both kept in plug flow during contact therebetween. It was disclosed there that a displacement of the water originally present in the substrate can be achieved such that a substantial fraction of the displaced water will be of relatively high purity, thereby reducing the quantity of the extractive fluid which is necessary, which, of course, reduces equipment costs and extractive fluid/water separation costs. The present invention relates to an improved method which utilizes the plug flow concept disclosed in the above-mentioned patent application.